RF MEMS switch technology has been introduced during the last 10-15 years as a prime candidate to replace the conventional GaAs FET and p-i-n diode switches in RF and microwave communication systems, mainly due to their low insertion loss, good isolation, linear characteristics and low power consumption. It has also provided the way for the development of novel revolutionary RF circuits that can be used in the next generation of broadband, wireless, and intelligent communication and radar systems. Recently, many researchers have been focusing on the development of miniaturized, low power and low cost, RF/Microwave circuits with RF MEMS switches. MEMS switches have been used in different RF circuit applications: tunable microwave filters, tunable phase shifters, tunable antennas and tunable matching networks. Microwave and millimeter-wave technology that offers wide tunability is essential for today's cost-driven commercial and military industries. In order to meet the above requirements, recently, micromachined tunable capacitors have been shown to have an adequate Q-factor when they are fabricated in either an aluminum or a polysilicon surface micromachining technology. Also, a three-plate structure with a wide tuning range has been reported.
Tunable capacitors are enabling components for millimeter-wave systems. There are two approaches to make such components. One is a compositional approach that improves properties of the materials, and the other is a physical approach that controls the gap or area of the dielectric layer for variable capacitance. MEMS switches' precise, micrometer-level movements are ideal drives for the physical approach. A MEMS-based switching diaphragm can be used as a variable capacitor. The tunability of this component is very impressive because an ultra-low loss, 2 μm air gap is used for the dielectric layer. However, the range of this variable capacitance is limited when the top member collapses onto the bottom plate.